1. Field of the Invention
The present invention pertains to a process and to a device for applying single or multiple coatings of a liquid coating material to a continuously moving backing web by the curtain coating process, in which a free-falling, vertical curtain of the coating material is formed and strikes the backing web along a wetting line. The present invention pertains in particular to a process for producing a heat-sensitive recording material.
2. Description of the Related Art
As a result of its numerous advantages, which include not only the possibility of applying several layers of different coating liquids simultaneously but also the superior surface properties of the applied coatings, the curtain coating process is firmly established as part of routine production in modern paper processing equipment. Curtain coating processes are often used to produce photo papers and magnetic recording papers, but the areas of application for the curtain coating process have been increasing, and they now include, for example, the production of heat-sensitive and pressure-sensitive recording material, ink-jet receiving media, and pigment-coated recording material, all of which can be considered firmly established prior art applications.
Among the known curtain coating processes, there are in particular two types which are especially important. First is the so-called slot coating process, in which the liquid coating material emerges through a downward-pointing outlet slot arranged transversely above the web-like base material to be coated. As the liquid leaves the slot, it immediately forms a free-falling liquid curtain. Second is the so-called slide-surface coating process. Here the liquid coating material is first applied to a downward-slanting slide, where it forms a thin film and flows down the slide under the effect of gravity, forming the free-falling liquid curtain at the end of the slide.
Independently of the type of design used to form the liquid curtain, there are numerous problems to be solved after the liquid curtain has formed. These involve, for example, the problem of stabilizing the liquid curtain and the problem of applying it as uniformly as possible to the continuously moving backing web.
One of these problems results in particular from the air carried along by the backing web when the attempt is made to operate at high speed. During the coating process, this air causes a permanent dynamic distortion of the wetting line. According to the invention, the “wetting line” is the line where the liquid curtain intersects the continuously moving web of base material. This line extends transversely across the backing web, and in the ideal case it is static and straight. Coating defects such as nonuniform thickness of the applied coating material and production breakdowns caused by the complete or even partial breaking-away of the liquid curtain are typical consequences of pronounced distortions of the wetting line.
Numerous proposals for solving this long-known and especially unpleasant problem have been made in the past. They have the goal in particular of stabilizing the liquid curtain by means of air suction devices such as that described in, for example, EP 0 489 978 B, which corresponds to U.S. Pat. No. 5,224,996. The air suction device is intended to have here the form of a concave air shield with various air chambers, which extend around a deflecting roller for the material web to be coated. The air shield is bounded by two end regions, at which various components such as brushes are used to increase the air resistance. The components are prevented from coming in direct contact with the material web because such contact would slow down production, i.e., a problem which could not be solved at that time.
Various air guide systems have already been proposed in the past to produce an air stream oriented against the direction of movement of the material web to be coated, i.e., an air stream which, beginning just in front of the wetting line, is applied directly above the material web to be coated to separate and to carry away the air entrained by the material web. All of these devices provide an air supply in the immediate vicinity of the liquid curtain and an air exhauster a certain distance farther away from the liquid curtain. Whereas U.S. Pat. No. 6,162,502 and, similarly, U.S. Pat. No. 5,624,715 implement this type of air guide system with concavely designed air shields extending around a material web being conveyed along a circular path, DE 198 08 159 A proposes an air shield which is set down on a material web being guided along a straight path.
According to a proposal of the general type in question in U.S. Pat. No. 3,369,522, a material web to be coated by a curtain coating process is deflected slightly downward between two guide rollers—one located upstream, the other and downstream of the wetting line—by a doctor located a certain distance upstream of the wetting line. The doctor forms an airtight shield for the air being carried by the material web toward the liquid curtain and is supposed to prevent more-or-less completely the interfering effects caused by the movements of air toward the liquid curtain. According to another embodiment, a hollow doctor, open at the bottom, is provided, which can also suction off the air.
Numerous experiments have shown the disadvantages of the known prior art in the form of coating defects such as skipping especially in the upper speed range above 1,200 m/min and especially clearly at speeds above 1,500 m/min. The task of the present invention is therefore to make available a process for applying single or multiple coatings of a liquid coating material to a continuously moving backing web by the curtain coating process and a device for implementing the process which still functions without difficulty even at very high production speeds.
To explain the previously described disadvantages of the prior art in question and also to overcome them, it was necessary, as part of the preparation of this document, to conduct studies to analyze the process of curtain coating as closely as possible:
In the process for applying single or multiple coatings by the curtain coating process, a very thin coating film is applied to a very fast and continuously moving backing web. When for this purpose the liquid coating material leaves the outlet slot of the slot coater or, so as not to be limited in any way in this respect, from the slide of the slide-surface coater, the actual curtain of coating material is formed, which then falls vertically downward under the force of gravity. At the moment the liquid coating material strikes the backing web along the wetting line, the vertical movement of this coating material is deflected in a direction which corresponds to the direction of movement of the backing web downstream of the wetting line, a movement which, according to the invention, preferably rises slightly. As the result of small vibrations of the backing web in the area of the wetting line, vibrations which are almost impossible for the human observer to see or feel and which are referred to in the present document as microvibrations, the surface of the backing web located under the arriving curtain moves at times faster vertically downward than the coating material in the curtain, especially when the web is traveling at very high speeds. As a result of these microvibrations, i.e., “microcycles”, skipping occurs at regular intervals, which means that the finished coating film has defects. If the coating material is a functional coating composition such as a heat-sensitive coating composition, which is preferably the case in the present document, the function of the coating film is considerably disturbed in the defect locations: the recording material thus produced is unusable.
The inventors were able to explain that the above-mentioned microvibrations are caused by the extremely rapid rises and falls of the backing web on the guide rollers known according to the prior art which serve as support means for the backing web downstream of the wetting line. This rising and falling, as the inventors discovered, is promoted by the rapid rotation of the guide rollers downstream of the wetting line. These rollers, designed with the typical diameters of 20-40 cm, draw in large amounts of air on their inlet side between the surface of the roller and the bottom surface of the backing web. A kind of air cushion forms, on which the backing web floats. This air cushion prevents the backing web from resting effectively on the guide rollers, and it can even cause the backing web to generate microvibrations when small surface irregularities on the guide rollers and/or the bottom surface of the backing web produce irregularities in the flow of air. On the outlet side, furthermore, not enough force is built up to press the backing web down onto the guide rollers. On the basis of these detailed observations and the accompanying interpretation of them, the inventors ultimately succeeded in solving the underlying problem.